Spalding : Absorption of atmospheric moisture 373 



i 



per cent, of their original weight when left in a desiccator over 

 night, and subsequently gained in two hours barely enough to 

 show that transpiration had been checked and absorption set up. 

 After further drying, and consequent departure from natural con- 

 ditions, the rate of gain became greater, but the experiments fail 

 to furnish any satisfactory evidence of the absorption of water 

 vapor from the atmosphere by the living parts of this plant before 

 pathological changes have set in. 



Experiments of the same kind with the creosote bush, CovilUa 

 tridetitata, were carried on both before and after heavy November 

 rains. The earlier experiments, in which branchlets and leaves 

 that appeared pretty dry, but still normally active, were employed, 

 exhibited a gain of nearly I per cent, in two and one half hours, 

 and their surface showed a distinct increase in the stickiness which 

 is due to the mixture of mucilaginous and waxy substances with 

 which they are coated. Later experiments with leaves and shoots 

 much washed by recent rains showed rapid loss of weight in a dry 

 atmosphere, amounting in some cases to ten per cent., while after 

 remaining in a nearly saturated atmosphere over night the gain was 

 only from one-tenth to less than two-tenths of what they had lost. 



These and other experiments with Covillea indicate on its part a 

 very limited capacity for absorption of water vapor from the atmos- 

 phere. None the less such absorption does take place, and it 

 seems highly probable that it is advantageous to the plant. Even 

 if absorption is so slight as merely to balance, or materially reduce 

 transpiration for a time, this may, at a critical period, prolong the 

 life and activity of leaves that would otherwise succumb ; and 

 furthermore the spread of the waxy-mucilaginous covering over 

 the leaves, which is manifestly favored by a high percentage of at- 

 mospheric moisture, must also materially aid in checking excessive 

 transpiration. % A period of high relative humidity, occurring as it 

 did the past summer, coincidently with a prolonged period of light 

 rainfall, would operate in both of these ways to prevent the prema- 

 ture loss of parts actively engaged in photosynthesis. It is ap- 

 parently impossible, at present, to express these complicated 

 relations quantitatively, but this by no means detracts from their 

 importance in the economy of a plant, which owes its dominant 

 position over wide areas not to a single advantageous structure but 

 to a complex of successful adaptations. 



